Hepatoprotective Dibenzocyclooctadiene and ... - ACS Publications

Article Cite This: J. Nat. Prod. XXXX, XXX, XXX−XXX

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Hepatoprotective Dibenzocyclooctadiene and Tetrahydrobenzocyclooctabenzofuranone Lignans from Kadsura longipedunculata Jiabao Liu,†,∥ Pankaj Pandey,‡,∥ Xiaojuan Wang,⊥ Xinzhu Qi,† Jiabao Chen,† Hua Sun,† Peicheng Zhang,† Yuanqing Ding,§ Daneel Ferreira,§ Robert J. Doerksen,‡ Mark T. Hamann,*,⊥ and Shuai Li*,† †

State Key Laboratory of Bioactive Substances and Functions of Natural Medicines, Institute of Materia Medica, Chinese Academy of Medical Sciences and Peking Union Medical College, Beijing 100050, People’s Republic of China ‡ Department of BioMolecular Sciences, Division of Medicinal Chemistry, and §Division of Pharmacognosy, Research Institute of Pharmaceutical Sciences, School of Pharmacy, University of Mississippi, University, Mississippi 38677-1848, United States ⊥ Department of Drug Discovery and Biomedical Sciences, College of Pharmacy, Medical University of South Carolina, Charleston, South Carolina 29425, United States S Supporting Information *

ABSTRACT: Five new dibenzocyclooctadiene lignans, longipedlignans A−E (1−5), five new tetrahydrobenzocyclooctabenzofuranones (6−10), and 18 known analogues (11−28) were isolated from the roots of Kadsura longipedunculata. Compounds 6− 10 are new spirobenzofuranoid−dibenzocyclooctadiene-type lignans. Their structures and absolute configurations were established using a combination of MS, NMR, and electronic circular dichroism data. Spirobenzofuranoids 6 and 15 showed moderate hepatoprotective activity against N-acetyl-p-aminophenol-induced toxicity in HepG2 cells with cell survival rates at 10 μM of 52.2% and 50.2%, respectively. longipedunculata, the roots of this plant were selected for investigation. As a result, 28 lignans were isolated, including five new dibenzocyclooctadiene lignans, longipedlignans A−E (1− 5), five new tetrahydrobenzocyclooctabenzofuranones (6−10), and 18 known analogues (11−28). Described herein are the isolation and structural elucidation of these new compounds and the determination of their absolute configurations through physicochemical and electronic circular dichroism (ECD) data analysis. In addition, the isolated compounds were assayed for their in vitro hepatoprotective effects.

Kadsura longipedunculata Finet et Gagnep. (Magnoliaceae) is a traditional Chinese medicine (TCM) plant, widely distributed in the southwest of China.1 The roots and stems have been used to treat rheumatoid arthritis, traumatic injury, irregular menstruation, canker sores, and gastrointestinal inflammation.2 Phytochemical studies have shown that the principal bioactive constituents of the genus Kadsura are lignans and triterpenoids. The lignans include the dibenzocyclooctadiene, cyclolignan, neolignan, and simple lignan classes, while the triterpenoids are mainly nortriterpenoids and those of the lanostane- and cycloartane-types. Benzofuranoid dibenzocyclooctadiene-type lignans of the genera Kadsura and Schisandra3 are of interest due to their biological activities, such as hepatoprotective,4−8 antioxidant,9,10 anti-HIV,6,11−15 and cytotoxic16,17 effects. In addition, spirobenzofuranodibenzocyclooctadiene-type lignans, characteristic constituents of the genus Kadsura, have significant antagonistic effects on platelet activating factor (PAF), which are regarded as the basis of promoting blood circulation to remove blood stasis.18 In order to investigate additional dibenzocyclooctadiene lignans with novel structures and bioactivities from K. © XXXX American Chemical Society and American Society of Pharmacognosy

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RESULTS AND DISCUSSION A 95% ethanol extract of the dried roots of K. longipedunculata was subjected to a combination of chromatographic steps on silica gel, Sephadex LH-20, and semipreparative HPLC to afford the new longipedlignans A−J (1−10) and 18 known lignans, heteroclitin D (11),19 heteroclitin H (12),20 schiarReceived: November 5, 2017

A

DOI: 10.1021/acs.jnatprod.7b00934 J. Nat. Prod. XXXX, XXX, XXX−XXX

Journal of Natural Products

Article

Table 1. 1H NMR Data of 1−5 in CDCl3 (δ in ppm, J in Hz, 500 MHz) position 6 7 8 9 6′ 7′α 7′β 9′ 10α 10β 2″ 3″ 4″ 5″ 6″ 7″ 8″ 9″ OCH3-3 OH-3′ OCH3-4′ OCH3-5′ a

1 6.57, 5.82, 1.96, 1.27, 6.60,

s s q (7.0) d (7.0) s

2

1.36, s 6.00, br s 5.96, br s

6.57, 5.84, 2.14, 1.24, 6.62, 2.93, 2.74, 1.38, 6.00, 5.96,

s s q (7.5) d (7.5) s d (13.5) d (13.5) s br s br s

7.33, 7.27, 7.43, 7.27, 7.33,

d (8.0) d (8.0) t (8.0) d (8.0) d (8.0)

7.38, 7.27, 7.44, 7.27, 7.38,

d (8.0) overlapped t (8.0) overlapped d (8.0)

3.87, 5.41, 3.30, 3.92,

s br s s s

3.83, 5.28, 3.41, 3.94,

s br s s s

2.80, br s

3 6.53, 5.72, 1.90, 1.24, 6.61,

4

s s q (7.0) d (7.0) s

6.55, 5.71, 2.08, 1.22, 6.58, 2.90, 2.71, 1.30, 6.00, 5.96, 5.83, 7.09,

2.77, br s 1.33, 6.00, 5.96, 5.72, 7.20,

s br s br s d (16.0) d (16.0)

5

s s q (7.0) d (7.0) s d (13.5) d (13.5) s br s br s d (16.0) d (16.0)

6.51, 5.55, 1.87, 1.23, 6.53,

s s overlapped d (7.0) s

2.72, br d (4.0) 1.30, s 6.00, br s 5.96, br s 5.92, br q (7.5) 1.88, d (7.0) 1.31, s

7.37,a br s

7.35,a br s

3.89, s

3.89, s

3.87, s

3.57, s 3.93, s

3.57, s 3.93, s

3.86, s 3.89, s

A broad singlet for H-5′′ to 9′′.

isanrin D (13),21 schiarisanrin C (14),21 schiarisanrin B (15),21 kadsulignan I (16),22 9-O-benzoyloxokadsuranol (17),23 9-Oisovaleroyloxokadsuranol (18),23 9-O-propanoyloxokadsuranol (19),23 heteroclitin Ia (20),24 heteroclitin E (21),19 heteroclitin Ib (22),25 binankadsurin A (23),26 7-O-acetylbinankadsurin A (24),27 heteroclitin P (25),28 kadsuphilol T (26),29 kadsuphilol B (27),30 and schisandlignan D (28)31 (Figure 1).The structures of the known compounds were determined by spectroscopic/spectrometric data analysis and comparison to literature values. Longipedlignan A (1) was assigned a molecular formula of C29H30O9 based on its HRESIMS (m/z 545.1789 [M + Na]+, calcd for 545.1782) and 13C NMR data. The UV spectrum with λmax values at 203 and 233 nm, together with the IR spectrum with absorption bands at 1613, 1584, 1509, and 1465 cm−1 (aromatic moiety), indicated the presence of a biphenyl moiety.23,32 The 1H NMR data (Table 1) displayed two aromatic singlets for a hexasubstituted biphenyl moiety at δH 6.57 (H-6) and 6.60 (H-6′) and three singlets for methoxy groups at δH 3.92 (3H), 3.87 (3H), and 3.30 (3H). A cyclooctadiene moiety was evident from two secondary methyl doublets at δH 1.36 (CH3-9) and 1.27 (CH3-9′), a methine at δH 1.96 (H-8), an oxymethine at δH 5.82 (H-7), and a methylene at δH 2.80 (H-7′α, β). The 13C NMR spectrum showed 29 resonances, including 18 carbons for three aromatic moieties, a methylenedioxy group at δC 101.5, three O-methyls at δC 56.1, 60.0, and 60.3, an ester carbonyl carbon at δC 165.1, two oxygenated carbons at δC 72.1 and 84.3, and four aliphatic carbons, including two methyl carbons at δC 17.0, 31.5, 46.0, and 47.1 (Table 2). The 1D and 2D NMR spectra of 1 were reminiscent of the presence of a dibenzocyclooctadiene lignan (Figure 2). The deshielded 1H and 13C NMR signals (Tables 1 and 2) and HMBC cross-peak of H-7 (δH 5.82) and C-1″ (δC 165.1) showed the presence of a benzoyloxy group at C-7. A secondary methyl singlet at δH 1.36 (CH3-9′) and HMBC cross-peaks of H-7′ (δH 2.80) with C-8′ (δC 72.1) and C-9′ (δC

Table 2. 13C NMR Data of 1−5 in CDCl3 (δ in ppm, 125 MHz)

B

position

1

2

3

4

5

1 2 3 4 5 6 7 8 9 1′ 2′ 3′ 4′ 5′ 6′ 7′ 8′ 9′ 10 OCH3-3 OCH3-4′ OCH3‑5′ 1″ 2″ 3″ 4″ 5″ 6″ 7″ 8″ 9″

134.3 119.1 141.1 136.4 149.4 102.8 84.3 47.1 17.0 131.5 117.6 147.0 134.6 151.3 107.7 46.0 72.1 31.5 101.5 60.0 60.3 56.1 165.1 129.3 129.2 128.4 133.2 128.4 129.2

134.1 118.7 141.2 136.5 149.3 103.0 81.5 49.2 17.3 132.1 117.5 147.0 134.2 151.1 107.2 48.2 75.0 24.3 101.4 59.9 60.5 56.1 165.7 129.6 129.5 128.2 132.9 128.2 129.5

134.4 118.8 141.0 136.1 149.1 102.6 83.3 46.8 16.8 131.5 117.5 146.7 134.4 150.8 107.5 45.8 71.8 31.2 101.3 59.7 60.5 55.9 165.1 116.8 144.8 129.3 128.8 128.1 130.3 128.1 128.8

134.2 118.5 141.1 136.3 149.1 102.8 80.7 49.0 17.1 132.1 117.4 146.7 135.0 150.6 107.0 48.0 74.9 24.0 101.3 59.8 60.6 55.8 165.7 117.4 144.5 130.3 128.8 128.1 133.9 128.1 128.8

134.6 118.3 140.5 135.8 148.9 102.2 83.1 46.5 16.6 131.3 117.1 146.5 134.2 150.7 107.3 45.6 71.4 30.8 101.0 59.4 60.1 55.7 165.2 125.7 140.7 15.5 19.7



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Figure 1. Structures of compounds 1−28.

urations of the remaining stereogenic centers of 1 were defined through ROESY correlations of H-6/H-7/H-8, H-8/CH3-9′, and H-6′/CH3-9, which were in agreement with a cyclooctadiene lignan possessing a twisted boat/chair conformation with a (7R, 8S, 8′R) absolute configuration.26 Thus, the structure of 1, longipedlignan A, and its (P, 7R, 8S, 8′R) absolute configuration were established as shown in Figure 1. The major structural difference between compounds 1 and 24 involves C-7 carrying benzoate and acetate moieties, respectively. The conformational analysis of compounds 1 and 24 using the OPLS336 force field and mixed torsional/low mode sampling via the MacroModel37 module implemented in the Schrödinger software yielded five and four low-energy conformers, respectively. These conformers were optimized further at the B3LYP/6-31G(d,p) and B3LYP/6-311+G(2d,p) levels. The Boltzmann population analysis using Gibbs free energies at the B3LYP/6-311+G(2d,p) level provided a total of three conformers, which showed >1% Boltzmann population in MeOH and in a vacuum for each compound. The timedependent density functional theory (TDDFT)38 calculation of the ECD for each conformer was performed at the B3LYP/6311+G(2d,p) level in the gas phase and in MeOH. The spectra were combined via Boltzmann weighting. The overall patterns of the calculated ECD spectra for compounds 1 and 24 were consistent with the experimental spectra, i.e., positive and negative Cotton effects (CEs) in the 200−220 and 240−260

Figure 2. Structure and key HMBC (red →) and ROESY (blue ↔) correlations of 1.

31.5), and CH3-9′ (δH 1.36) with C-7′ (δC 46.0), C-8′ (δC 72.1), and C-8 (δC 47.1), indicated that C-8′ is an oxygenated tertiary carbon. The ECD curve of 1 displayed sequential negative and positive Cotton effects near 250 and 210 nm, respectively. The so-called A band near 250 nm is associated with the in-phase combination of the 1La transitions polarized along the long axis of the biphenyl chromophore.33 Such a negative A band in the ECD spectrum of 1 consequently reflects a P-biphenyl absolute configuration.34,35 The exciton coupling was supported by the UV absorption maximum at 233 nm. The absolute configC



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Figure 3. Experimental ECD spectrum (black), and the calculated ECD spectra in MeOH (red) of (A) the lowest-energy conformer 1a of compound 1 and (B) the lowest-energy conformer 24a of compound 24. The σ-value (artificial line broadening) was set to 0.29 eV.

Figure 4. Molecular orbitals involved in key transitions in the calculated ECD spectrum of the lowest-energy conformer 1a at the B3LYP/6311+G(2d,p) level in MeOH. Orbitals are plotted with a 0.05 e−/au3 isovalue.

nm regions, respectively. Figure 3A depicts overlays of the experimental spectrum of compound 1 and of the computed

spectrum of the lowest-energy conformer 1a of compound 1 in MeOH at the B3LYP/6-311+G(2d,p) level, while Figure 3B D



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Figure 5. Molecular orbitals involved in key transitions in the calculated ECD spectrum of the lowest-energy conformer 24a at the B3LYP/6311+G(2d,p) level in MeOH. Orbitals are plotted with a 0.05 e−/au3 isovalue.

Information) occurred at 223.2 nm, due to the electronic transition from MO138 (HOMO) to MO147, which corresponds to electron delocalization from ring A and the 1,3-dioxolane moiety to ring B. Furthermore, the positive CE for compound 24 around 205 nm could be assigned to an excitation with positive rotatory strength at a wavelength of 217.88 nm, for the electronic transition from MO116 to MO124, which corresponds to π → π electron excitation in the B-ring (Figure 5). Longipedlignan B (2) also gave a molecular formula of C29H30O9 from its HRESIMS (m/z 545.1781 [M + Na]+, calcd for 545.1782) and 13C NMR data. The UV, IR, ECD, and NMR data of 2, highly similar to those of 1, suggested the presence of a (P) biphenyl-configured dibenzocyclooctadiene lignan.34 However, the 1H NMR spectrum of 2 showed two C7′ gem-proton doublets at δH 2.93 and 2.74 (H-7′, d, J = 13.5 Hz), instead of the two-proton singlet (δH 2.80) in 1. Additionally, C-8′ (δC 75.0) of 2 resonated at a lower field relative to C-8′ (δC 72.1) of 1, and the shielding of CH3-9′ (δC 24.3) of 2 relative to 1 (δC 31.5) indicated a different configuration at C-8′, which was confirmed by the ROESY correlations of H-7″/CH3-9′ and H-6′/CH3-9′ (magenta arrows in Figure 6). Combined with the ROESY correlations

shows such results for the lowest-energy conformer 24a of compound 24. The calculated spectra of compounds 1 and 24 in MeOH are consistent with the experimental spectra; therefore, the absolute configurations of these compounds could be confirmed unequivocally. The Boltzmann-weighted averaged spectra of conformers 1a−1c (compound 1) and 24a−24c (compound 24) in MeOH and in the gas phase are shown in Figure S1 in the Supporting Information. Considering the extended π-system of compound 1, including the biphenyl moiety and benzoate chromophores, the negative CE near 250 nm in the experimental ECD spectrum is characteristic for this molecule. This was supported by analysis of the molecular orbitals (MOs) involved in the key transitions that contributed the most to the ECD spectrum of the lowest-energy conformers 1a (62.20% Boltzmann population) (Figure S2 (A), Supporting Information) and 24a (47.12%) (Figure S2 (B), Supporting Information), respectively, at the B3LYP/6311+G(2d,p) level in MeOH. MO analysis of conformer 1a (Figure 4) suggested that the negative CE at 245 nm in the experimental spectrum could be ascribed to the excitations with negative rotatory strengths at wavelengths of 248.16, 247.31, 245.27, and 241.10 nm (Table S1, Supporting Information), which were caused by the electronic transitions from MO131 to MO139 (LUMO), MO137 to MO140, MO138 (HOMO) to MO144, and MO138 (HOMO) to MO144, respectively. Among these (Table S1, Supporting Information), the predominant electronic transition was that from MO138 (HOMO) to MO144, which corresponds to electron delocalization from rings A and B and the 1,3-dioxolane moiety to ring B (Figure 4). Similarly, MO analysis of conformer 24a (47.12%) (Figure 5) suggested that the negative CE at 253 nm in the experimental spectrum could be ascribed to the excitations with negative rotatory strengths at wavelengths of 268.13, 259.29, and 242.81 nm, which were generated via the electronic transitions from MO118 (HOMO) to MO119 (LUMO) and from MO116 to MO119 (LUMO), respectively. Between these, the predominant electronic transition was that from MO116 to MO119 (LUMO), which corresponds to electron delocalization from ring B to rings A and B (Figure 5). In addition, the positive CE for compound 1 around 205 nm could be assigned to excitations with positive rotatory strengths at wavelengths of 234.27, 232.79, 229.66, and 223.2 nm, which were for the electronic transitions from MO133 to MO139 (LUMO), from MO137 to MO144, from MO132 to MO139 (LUMO), and from MO138 (HOMO) to MO147, respectively (Figure 4). Among these, the predominant CE based on oscillatory and rotatory strength (Table S1, Supporting

Figure 6. Structure and key ROESY correlations (blue ↔) and characteristic ROESY correlations (magenta ↔) of 2.

of H-6/H-7/H-8, H-6′/CH3-9, and CH3-9′/H-7′, this indicated that the 7-benzoate group, CH3-9, and CH3-9′ are α-oriented (Figure 6). Thus, the structure of 2, longipedlignan B, and its (P, 7R, 8S, 8′S) absolute configuration were defined as shown (Figure 1). The molecular formulas of longipedlignans C (3) and D (4) were determined as C31H32O9 by 13C NMR spectroscopic data and HRESIMS analyses (m/z 571.1951 [M + Na]+, calcd for 571.1939, and m/z 571.1936, calcd for 571.1939, respectively). E



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Table 3. 1H NMR Data of 6−10 in CDCl3 (δ in ppm, J in Hz, 500 MHz) position 6 7 8 9 6′ 7′α 7′β 8′ 9′ 10α 10β 11α 11β OCH3-4′ OCH3-5′ 2″ 3″ 4″ 5″ 6″ 7″ 8″ 9″

6 6.48, 5.76, 1.94, 1.33, 6.37,

s s br q (7.0) d (7.0) s

2.66, br s 1.34, 6.04, 5.97, 4.64, 4.14, 3.88, 3.10,

s d d d d s s

7.72, 7.37, 7.54, 7.37, 7.72,

d (8.0) t (8.0) t (8.0) t (8.0) d (8.0)

(2.0) (2.0) (8.0) (8.0)

7 6.52, 5.77, 2.08, 1.28, 6.26, 2.75, 2.64,

s s m d (7.0) s d (13.0) d (13.0)

1.58, 6.03, 5.96, 4.62, 4.12, 3.95, 3.08,

s br s br s d (8.0) d (8.0) s s

7.82, 7.36, 7.52, 7.36, 7.82,

d (8.0) t (8.0) t (8.0) t (8.0) d (8.0)

8

9

10

6.30, 5.95, 1.76, 1.02, 6.24, 4.08,

s overlapped m d (7.0) s overlapped

6.32, 5.96, 1.78, 0.98, 6.27, 4.10,

s overlapped m d (6.5) s d (9.5)

6.36, 6.05, 1.82, 1.04, 6.27, 4.12,

s d (7.0) m d (7.0) s d (9.5)

2.01, 0.90, 6.02, 5.97, 5.08, 4.62, 3.78, 4.08, 1.79,

m d (7.0) br s br s d (8.5) d (8.5) s s s

2.04, 0.90, 6.02, 5.97, 5.08, 4.62, 3.77, 4.07, 2.19, 1.02, 1.02,

m d (7.5) br s br s d (8.5) d (8.5) s s m d (7.0) d (7.0)

2.08, 0.95, 6.03, 5.98, 5.11, 4.64, 3.72, 4.03, 6.13, 7.57,

m d (7.0) br s br s d (8.5) d (8.5) s s d (16.0) d (16.0)

7.54, m 7.36, m 7.54, m

The 13C NMR spectra revealed the presence of an (E)cinnamoyl group at δC 165.1, 116.8, 144.8, 129.3, 128.8 (2C), 128.1 (2C), and 130.3 for 3 and δC 165.7, 117.4, 144.5, 130.3, 128.8 (2C), 128.1 (2C), and 133.9 for 4. The HMBC crosspeaks from H-7 (δH 5.72 and 5.71 for 3 and 4, respectively) to the ester carbonyl carbons (δC 165.1 and 165.7 for 3 and 4, respectively) placed the (E)-cinnamoyl group at C-7. Compounds 3 and 4 also had opposite configurations at C8′, as shown by comparison of the chemical shifts of C-8′ and ROESY correlations. The P-absolute configurations of the biphenyl moieties in 3 and 4 were again assigned based on the ECD data.26,34 Thus, the structures of 3 and 4, longipedlignans C and D, were established as shown in Figure 1. The molecular formula of longipedlignan E (5) was determined as C27H32O9 via the HRESIMS ion at m/z 523.1942 [M + Na]+ (calcd for 523.1939) and 13C NMR data. The NMR data of 5 showed similarities to those of 2. However, a C-7 angeloyloxy group (δC 165.2, 125.7, 140.7, 15.5, and 19.7) in 5 replaced the (E)-cinnamate moiety in 4. This was confirmed by the HMBC cross-peak from H-7 (δH 5.55) to the ester carbonyl carbon (δC 165.2). The P-absolute configuration of the biphenyl moiety in 5 was assigned based on the ECD, UV, and IR data. ROESY correlations of H-6/H8/H-7, H-7/CH3-9′, and H-6′/CH3-9 of 5 indicated that CH39 is α-oriented and that H-7 and CH3-9′ are β-oriented.26,34 Thus, the structure of 5, longipedlignan E, and its (P, 7R, 8S, 8′R) absolute configuration were established as shown in Figure 1. Longipedlignans F (6) and G (7) were determined to have the same molecular formulas of C29H28O9 from their HRESIMS (m/z 521.1806 and 521.1818 [M + H]+, respectively) 1H and 13 C NMR data (Tables 3 and 4). The 13C NMR and HSQC spectra of 6 and 7 exhibited 18 carbon atoms including an α,βunsaturated carbonyl group (δC 194.6 and 194.7, respectively), a pentasubstituted aromatic moiety, four olefinic carbons (δC 133.2, 156.1, 121.8, 144.7 and 132.6, 155.9, 122.2, 144.3,

Table 4. 13C NMR Data of 6−10 in CDCl3 (δ in ppm, 125 MHz)

F

position

6

7

8

9

10

1 2 3 4 5 6 7 8 9 1′ 2′ 3′ 4′ 5′ 6′ 7′ 8′ 9′ 10 11 OCH3-4′ OCH3-5′ 1″ 2″ 3″ 4″ 5″ 6″ 7″ 8″ 9″

128.9 118.9 144.2 130.4 150.7 101.6 83.2 47.8 17.9 144.7 66.1 194.6 133.2 156.1 121.8 44.7 72.9 31.5 102.2 79.4 59.0 58.3 165.6 130.1 131.7 128.3 129.5 129.5 128.3

129.1 118.1 144.3 131.1 150.7 101.8 80.6 50.1 17.9 144.3 66.7 194.7 132.6 155.9 122.2 46.7 75.6 24.7 102.1 79.5 59.0 58.7 166.0 130.4 128.1 129.8 131.7 129.8 128.1

127.8 122.1 144.6 130.3 150.2 101.0 76.5 42.1 9.0 146.2 63.7 197.8 132.7 155.6 124.3 82.0 38.9 19.2 102.0 79.5 59.2 59.1 170.0 20.3

128.0 121.9 144.6 130.3 150.2 101.3 76.2 42.3 9.0 146.0 63.8 195.5 132.9 155.4 124.2 82.0 38.7 19.2 102.0 79.5 59.2 58.9 176.7 33.3 19.9 18.3

130.4 122.4 145.9 130.6 150.5 101.3 77.9 42.8 9.5 146.2 64.0 195.6 133.0 155.9 124.8 82.2 39.0 19.4 102.2 79.9 59.5 59.4 166.6 117.5 146.2 134.8 128.6 128.9 130.4 128.9 128.6



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Cotton effects near 370 and 320 nm. Except for a contribution by Li and Xue23 that credited these CEs as emanating from the “spirobenzofuranoid” moiety, their presence has largely been ignored in other reports. Li and Xue23 additionally interpreted CEs in the 230−270 nm region in terms of exciton chirality involving the benzoate and dienone chromophores to assign the C-7 absolute configuration of their oxokadsurane derivatives. It is debatable whether the phenomenon of exciton chirality is applicable to two chromophores as distinct as these benzoate and cyclohexa-2,4-dienone moieties. It should also be noted that the CEs near 370 and 320 nm in the ECD spectra of 6 and 8 (Figure 9) are conspicuously absent in the spectra of 1−5 (Figure 3, Figures S10, S15, S22, S29, and S36, Supporting Information). Such inconsistencies in relating the observed CEs to the absolute configuration of the spiro carbon atoms of the tetrahydrobenzocyclooctabenzofuranone core prompted recourse to comparison of the experimental and computed ECD spectra of this unique class of lignans. In order also to assess whether the C-7 benzoate group in 6 indeed participates in exciton coupling with the cyclohexa-2,4-dienone moiety, the ECD spectra of compounds 6 and 8 (containing a C-7 acetate unit) were computed and compared to the experimental spectra.20 The conformational analysis of compounds 6 and 8 was done using the MacroModel37 program implemented in the Schrödinger software and resulted in eight and six low-energy conformers, respectively. Geometry optimization along with frequency calculations using hybrid DFT at the B3LYP/6311+G(2d,p) level yielded four conformers for each of compounds 6 and 8 that showed >1% Boltzmann population in MeOH; however, in a vacuum compounds 6 and 8 retained five and four conformers, respectively. These conformers were used for calculating the ECD spectra using TDDFT38 with the B3LYP/6-311+G(2d,p) method in a vacuum and in MeOH. The experimental ECD spectra corresponded to the calculated ECD spectra in MeOH (Figure 9 and Figure S3, Supporting Information); therefore, further calculations involved the conformers present in MeOH. The calculated averaged spectra of compounds 6 and 8 in MeOH were in accordance with the experimental spectra. Thus, the (7R, 8S, 2′S, 8′R) and (7R, 8R, 2′S, 7′R, 8′S) absolute configurations of compounds 6 and 8, respectively, were confirmed. MO analysis (Figure 10) of the lowest energy conformer 6c of 6 [Figure S2 (C), Supporting Information] suggested that the positive CE at 371.5 nm in the experimental ECD spectrum could be ascribed to the excitation with a positive rotatory strength at 383.73 nm, which was caused by the electronic transition from MO137 (HOMO) to MO138 (LUMO). The negative CE at 319 nm could be assigned to excitations having negative rotatory strengths at 354.35 and 327.51 nm, which correlated with the electronic transitions from MO136 to MO138 and from MO134 to MO138, respectively. These electronic transitions involved the carbonyl lone pair electrons (MO134) and excitation from ring A, ring B, and the 1,3dioxolane ring (MO136) to ring B (MO138). MO analysis (Figure 11) of the lowest energy conformer 8b of 8 [Figure S2 (D), Supporting Information] suggested that the positive CE at 367.5 nm in the experimental spectrum arose from the excitation with positive rotatory strength at 353.86 nm, due to the electronic transition from MO120 to MO122 (LUMO), an electronic transition involving a π → π* transition involving the B-ring. The negative CE at 315.5 nm could be assigned to an excitation having a negative rotatory strength at

respectively), three methines (δC 121.8, 47.8, 83.2 and 122.2, 50.1, 80.6, respectively), two methylenes (δC 79.4, 44.7 and 79.5, 46.7, respectively), and two secondary methyls (δC 31.5, 17.9 and 24.7, 17.9, respectively) for a C18 framework. The presence of characteristic AB quartets at δH 4.64 and 4.14 (each d, J = 8.0 Hz) and 4.62 and 4.12 (each d, J = 8.5 Hz) in the 1H NMR spectra of 6 and 7 (Table 3), respectively, combined with the presence of quaternary carbons in the 13C NMR spectrum of 6 and 7 (δC 66.1 and 66.7, respectively) suggested that 6 and 7 are modified dibenzocyclooctadiene-type lignans with an added spirobenzofuranoid moiety.35 Comparison of the spectroscopic data of 6 and 7 revealed these two compounds to be structurally quite similar, except for the C-8′ absolute configuration. Analysis of the ROESY correlations, especially the correlation of H-6′/H-9′ in 7 (magenta arrow in Figure 7),

Figure 7. Comparison of key ROESY (blue ↔) correlations of 6 and 7 and characteristic ROESY correlations (magenta ↔) of 7.

and comparison of the 1H and 13C NMR data of 6 and 7 (Tables 3 and 4) revealed that CH3-9′ is β- and α-oriented in 6 and 7, respectively.24 ROESY correlation between H-4″/6″ of the benzoate group and OCH3-5′ and HMBC correlation between H-7 (δH 5.76 and 5.77) and C-1″ (δC 165.6 and 166.0) imply that the benzoate group is located at C-7. This is in good agreement with the shifting of the OCH3-5′ resonance to a higher field (δH 3.10 and 3.08) because of the anisotropic shielding effect of the benzene ring.21 A literature survey revealed that compounds of types 6−21 have been classified as “tetrahydrofuranoid dibenzocyclooctadiene” lignans.19−24 Unsurprisingly, such an erroneous classification also led to the interpretation of the ECD data as if these compounds still possess a chiral biphenyl chromophore.19−24 In fact, the tetracyclic core (29, Figure 8) of compounds 6−21 is a 1,2,3,4-tetrahydro-8H,9H-benzo[1,8]cycloocta[1,2,3-cd]benzofuran-8-one moiety, which will be abbreviated as “tetrahydrobenzocyclooctabenzofuranone” in the subsequent discussion of the present contribution. The ECD spectra (Figure 9) of compounds 6 and 8 are dominated by high-amplitude sequential positive and negative

Figure 8. Tetracyclic core of compounds 6−10. G



Article

Figure 9. Experimental ECD spectrum (black) and the calculated ECD spectra in MeOH (red) of (A) the lowest-energy conformer 6c of compound 6 and (B) the lowest-energy conformer 8b of compound 8. The σ-value (artificial line broadening) was set to 0.29 eV.

CH3-9, and from CH3-9′ to CH3-9. Therefore, the structure of 8, longipedlignan H, was defined as shown. Longipedlignans I (9) and J (10) were assigned molecular formulas of C26H30O9 and C31H30O9, according to their HRESIMS (m/z 509.1793 [M + Na]+ and 569.1788 [M + Na]+, respectively) and 13C NMR spectroscopic data. The NMR data of 9 and 10 were similar to those of 8, except for changes involving the C-7 substituents. The C-7 acetoxy group in 8 was replaced by an isobutanoate group in 9 and by an (E)cinnamate group in 10. The ECD and ROESY data of 9 and 10 both suggested the same (7R, 8R, 2′S, 7′R, 8′S) absolute configurations of these new tetrahydrobenzocyclooctabenzofuranone lignans. Thus, the structures of longipedlignans K (9) and L (10) were formulated as shown in Figure 1. All the compounds were evaluated for their in vitro hepatoprotective activity against N-acetyl-p-aminophenol (APAP)-induced toxicity in HepG2 (human hepatocellular liver carcinoma cell line) cells, using the hepatoprotective drug bicyclol as the positive control, and moderate hepatoprotective activity was observed (Table S3, Supporting Information). Compounds 6 and 15 effected a cell survival rate of 52.2% and 50.2% (cf. bicyclol, 49.0%), respectively, at 10 μM when added into resuscitated HepG2 cells incubated with APAP for 48 h.

329.12 nm, which matched the electronic transition from MO118 to MO122 (LUMO), involving the lone pair electrons of the carbonyl group (ring B) and the π system of the B-ring. The negative CE at 246 nm in the experimental spectrum of compound 6 could be ascribed to the excitation with negative rotatory strength at 256.62 nm, which was caused by the electronic transition from MO133 to MO139 (LUMO+1). This electronic transition corresponded to a π → π* transition in the benzoate moiety. The negative CE at 220 nm could be assigned to an excitation with negative rotatory strength at 229.18 nm, based on the electronic transition from MO135 to MO142, which corresponded to a π → π* transition in the Aring. On the other hand, the positive CE for compound 8 at 246 nm in the experimental spectrum could be ascribed to an excitation with positive rotatory strength at 248.77 nm, caused by the electronic transition from MO116 to MO122 (LUMO). This transition corresponds to an n → π* transition involving the 7′-hydroxy group and the cyclohexa-2,4-dienone moiety. The negative CE at 221 nm could be assigned to excitations with negative rotatory strengths at wavelengths of 230.37, 218.93, 215.41, and 214.29 nm, corresponding to electronic transitions from MO119 to MO124, from MO120 to MO126, from MO119 to MO127, and from MO118 to MO123, respectively. Among these rotatory strengths, the predominant CE based on oscillatory and rotatory strengths (Table S2, Supporting Information) occurred at 215.41 nm, involving the π-electrons of the trioxygenated A- and B-rings. Longipedlignan H (8) was assigned a molecular formula of C24H26O9 from the HRESIMS ion at m/z 481.1482 [M + Na]+ and the 13C NMR data. The UV, IR, NMR, and ECD data showed that 8 is a (7R,8R,2′S,7′R,8′S)-tetrahydrobenzocyclooctabenzofuranone lignan.19,20 The 13C NMR spectrum exhibited 19 carbon atoms, including six aromatic, four olefinic, a ketocarbonyl, a quaternary carbon, an oxymethylene, four methines, and two secondary methyl carbons for the tetrahydrobenzocyclooctabenzofuranone-type core, in addition to a methylenedioxy group, two methoxy groups, and an acetyl group. This was confirmed by 1H−1H COSY correlations of H7′/H-8′/H-8/H-7, H-8′/CH3-9′, and H-8/CH3-9 (Figure 12). An HMBC cross-peak of the oxymethine proton at δH 5.95 (H7) with C-1″ (δC 170.0) showed that the acetoxy group is connected to C-7 (Figure 12). The acetoxy group, H-7′, CH39′, and CH3-9 are α-oriented on the basis of the ROESY correlations from H-6 to H-7 and H-8, from H-6′ to H-7′ and

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EXPERIMENTAL SECTION

General Experimental Procedures. Optical rotations were measured on a JASCO P-2000 polarimeter (JASCO Inc., Easton, MD, USA), and UV spectra with a JASCO V-650 spectrophotometer. ECD spectra were recorded on a JASCO J-815 spectrometer, and IR spectra on a Nicolet 5700 spectrometer (Thermo Electron Corporation, Madison, WI, USA) using an FT-IR microscope transmission method. The 1H and 13C NMR spectra were recorded on INOVA-500 (Varian, Inc., Palo Alto, CA, USA) and Bruker AV500III spectrometers (Bruker, Billerica, MA, USA). Chemical shifts are given in δ (ppm) values relative to those of the solvent signal [CDCl3 (δH 7.26; δC 77.2)]. The standard pulse sequences programmed into the instrument were used for each 2D measurement. HRESIMS data were acquired using an Agilent 6520 Accurate-Mass Q-Tof LC/MS mass spectrometer (Agilent Technologies, Waldbronn, Germany). Analytical reversed-phase HPLC was performed on a COSMOSIL 5C18-PAQ Waters column (4.6 i.d. × 250 mm, Waters, Nacalai, San Diego, CA, USA) eluted with H2O−MeOH (flow rate, 1 mL/min; 220 nm UV detection) at room temperature. Preparative RP-HPLC was performed on a COSMOSIL 5C18-PAQ Waters column (250 × 10 mm, 5 μm) at room temperature. Column chromatography was performed with silica gel (40−63 μm; Silicycle, Quebec City, QC, Canada), C18 120 Å reversed-phase silica gel (RP-18; 50 μm; Silicycle), and Sephadex LH-20 (GE Healthcare Bio-Science AB, Uppsala, H



Article

Figure 10. Molecular orbitals involved in the key transitions in the calculated ECD spectrum of the lowest-energy conformer 6c at the B3LYP/6311+G(2d,p) level in MeOH. Orbitals are plotted with a 0.05 e−/au3 isovalue. column chromatography (CHCl3−MeOH, 200:1, 100:1, 50:1 and MeOH) to afford subfractions 12.1−12.5. Fraction 12.1 (36 g) was chromatographed on silica gel (n-hexane−EtOAc, 1:9−1:1) to give four subfractions, 12.1.1−4. Fraction 12.1.2 (10.9 g) was subjected to RP-18 column chromatography (65% MeOH−H2O) to afford five subfractions, 12.1.2.1−5. Fraction 12.1.2.3 (2.5 g) was purified by preparative HPLC (60% MeOH−H2O) to give five fractions, 12.1.2.3A−E. Fraction 12.1.2.3A (80 mg) was further separated by semipreparative HPLC (58% MeOH−H2O) to give 12 (10 mg) and 19 (2 mg). Fraction 12.1.2.3B (100 mg) was chromatographed by semipreparative HPLC (55% MeOH−H2O) to give 13 (6 mg) and 20 (47 mg). Fraction 12.1.2.3C (160 mg) was repeatedly subjected to semipreparative HPLC (58% MeOH−H2O and 45% CH3CN−H2O) to give 11 (19 mg), 21 (5 mg), 23 (10 mg), and 28 (13 mg). Fraction 12.1.2.3D (180 mg) was subjected to semipreparative HPLC (45% CH3CN−H2O) to give 1 (5 mg), 25 (2 mg), 27 (5 mg), 18 (30 mg),

Sweden). Fractions were monitored by TLC, and spots were visualized by heating silica gel plates sprayed with 10% H2SO4 in EtOH. Plant Material. The roots of Kadsura longipedunculata were collected in Jiujiang County of Jiangxi Province, People’s Republic of China, in March 2010, and identified by Ce-Ming Lin, Institute of Biology Resources, Jiangxi Academy of Science. A voucher specimen (ID-S-2428) is deposited in the herbarium of the Institute of Materia Medica, Chinese Academy of Medical Science (CAMS) and Peking Union Medical College, Beijing. Extraction and Isolation. The air-dried roots of K. longipedunculata (34 kg) were extracted with EtOH−H2O (95:5, v/v) at room temperature, and the solution concentrated in vacuo to yield a homogenate (2.4 kg), which was chromatographed on a silica gel column, eluting with a petroleum ether−acetone gradient system (50:1, 10:1, 5:1, 3:1, 1:1), acetone, and 80% EtOH, successively, to give fractions 1−14. Fraction 12 (146.8 g) was subjected to silica gel I



Article

Figure 11. Molecular orbitals involved in the key transitions in the calculated ECD spectrum of the lowest-energy conformer 8b at the B3LYP/6311+G(2d,p) level in MeOH. Orbitals are plotted with a 0.05 e−/au3 isovalue. and 26 (10 mg). Fraction 12.1.3 (16.0 g) was subjected to separation over a silica gel column (CHCl3−MeOH, 80:1−40:1) to give seven subfractions, 12.1.3A−G. Fraction 12.1.3C (880 mg) was further subjected to preparative HPLC (58% MeOH−H2O) to give five fractions, 12.1.3C1−C5. Fraction 12.1.3C2 (120 mg) was separated by semipreparative HPLC (53% MeOH−H2O and 45% CH3CN−H2O) to give 3 (10 mg), 5 (10 mg), 17 (5 mg), and 24 (4 mg). Fraction 12.1.3C4 (340 mg) was further separated by semipreparative HPLC (50% MeOH−H2O and 45% CH3CN−H2O) to give 22 (43 mg). Fraction 12.1.3D (1.03 g) was subjected to preparative HPLC (55% MeOH−H2O) to give three subfractions, 12.1.3D1−12.1.3D3. Fraction 12.1.3D1 (51 mg) was separated by semipreparative HPLC (50% MeOH−H2O) to give 14 (10 mg) and 16 (6 mg). Fraction 12.1.3D2 (89 mg) was purified further by semipreparative HPLC

Figure 12. Structure and key 1H−1H COSY (bold), HMBC (red →) and ROESY (blue ↔) correlations of 8. J



Article

(320); IR (KBr) νmax 3463, 2983, 2937, 2853, 1721, 1649, 1576, 1503, 1487, 1451, 1395, 1261, 1210, 1128, 1094, 1066, 1025, 954, 935, 717 cm−1; 1H and 13C NMR data, see Tables 3 and 4; ESIMS (+) m/z 543 [M + Na]+; HRESIMS (+) m/z 521.1818 [M + H]+ (calcd for C29H28NaO9, 521.1806). (7R,8R,2′S,7′R,8′S)-Longipedlignan H (8): yellow, amorphous powder; [α]20 D +35 (c 0.1, MeOH); UV (MeOH) λmax (log ε) 220 (1.95), 275 (0.56), 320 (0.58) nm; ECD (c 0.1, MeOH) [θ] (nm) +17 × 103 (368), −26 × 103 (316), +12 × 103 (246), −40 × 103 (222); IR (KBr) νmax 3450, 2963, 2930, 2881, 1737, 1649, 1585, 1503, 1489, 1440, 1387, 1263, 1235, 1123, 1098, 1064, 1024, 972, 933 cm−1; 1H and 13C NMR data, see Tables 3 and 4; ESIMS (+) m/z 481 [M + Na]+; HRESIMS (+) m/z 481.1482 [M + Na]+ (calcd for C24H26NaO9, 481.1469). (7R,8R,2′S,7′R,8′S)-Longipedlignan I (9): yellow, amorphous powder; [α]20 D +23 (c 0.1, MeOH); UV (MeOH) λmax (log ε) 219 (1.37), 327 (0.14) nm; ECD (c 0.5, MeOH) [θ] (nm) +0.5 × 103 (368), −0.7 × 103 (317), +0.4 × 103 (250), −1.4 × 103 (222); IR (KBr) νmax 3470, 2971, 2934, 2879, 1732, 1649, 1585, 1503, 1489, 1458, 1388, 1304, 1265, 1243, 1193, 1121, 1064, 1027, 995, 965, 934 cm−1; 1H and 13C NMR data, see Tables 3 and 4; ESIMS (+) m/z 509 [M + Na]+; HRESIMS (+) m/z 509.1793 [M + Na]+ (calcd for C26H30NaO9, 509.1782). (7R,8R,2′S,7′R,8′S)-Longipedlignan J (10): yellow, amorphous powder; [α]20 D +6.2 (c 0.5, MeOH); UV (MeOH) λmax (log ε) 202 (1.31), 218 (1.55), 271 (1.12) nm; ECD (c 0.5, MeOH) [θ] (nm) −0.7 × 103 (314), +1.3 × 103 (277), −0.7 × 103 (245), −1.6 × 103 (216); IR (KBr) νmax 3382, 2958, 2923, 2852, 1718, 1662, 1628, 1472, 1419, 1370, 1272, 1249, 1198, 1150, 1115, 1094, 1054, 999, 933, 903, 802 cm−1; 1H and 13C NMR data, see Tables 3 and 4; ESIMS (+) m/z 569 [M + Na]+; HRESIMS (+) m/z 569.1788 [M + Na]+ (calcd for C31H30NaO9, 569.1782). Hepatoprotective Activity Assay. All the compounds were tested for hepatoprotective activity against APAP-induced toxicity in HepG2 cells by means of a published MTT method.39 Bicyclol was used as a positive control substance.

(54% MeOH−H2O) to give 5 (15 mg) and 15 (4 mg). Fraction 12.1.3D3 (20 mg) was purified by semipreparative HPLC (55% MeOH−H2O) to give 7 (5 mg) and 10 (5 mg). Fraction 12.5 (10.0 g) was subjected to passage over a Sephadex LH-20 column (CHCl3− MeOH, 1:1) and semipreparative HPLC (60% MeOH−H2O), sequentially, to give 8 (13 mg). Fraction 13 (52.2 g) was subjected to RP-18 column chromatography (MeOH−H2O, 40:60−90:10) to afford three subfractions, 13.1−13.3. Fraction 13.2 (18.0 g) was separated by silica gel column chromatography (CHCl3−MeOH, 60:1, 50:1, 30:1 and MeOH) to afford five subfractions. Fraction 13.2.1 (3.3 g) was subjected to Sephadex LH-20 column chromatography (CHCl3−MeOH, 1:1) to give five subfractions, 13.2.1.1−5. Fraction 13.2.1.3 (120 mg) was separated by semipreparative HPLC (60% MeOH−H2O) to afford 2 (5 mg), 17 (5 mg), and 19 (4 mg). Fraction 13.2.1.4 (54 mg) was purified by semipreparative HPLC (60% MeOH−H2O) to afford 9 (6 mg). Fraction 13.2.1.4 (34 mg) was purified by semipreparative HPLC (55% MeOH−H2O) to afford 6 (13 mg). Fraction 13.2.1.8 (10 mg) was purified by semipreparative HPLC (59% MeOH−H2O) to afford 4 (3 mg). (P,7R,8S,8′R)-Longipedlignan A (1): yellow, amorphous powder; [α]20 D −11 (c 0.2, MeOH); UV (MeOH) λmax (log ε) 203 (2.01), 233 (1.94) nm; ECD (c 0.2, MeOH) [θ] (nm) −2.5 × 103 (254), +1.3 × 103 (209); IR (KBr) νmax 3575, 3448, 2978, 2940, 2842, 1723, 1613, 1584, 1509, 1465, 1429, 1374, 1341, 1271, 1250, 1140, 1105, 1072, 1051, 1013, 975, 937, 830, 714 cm−1; 1H and 13C NMR data, see Tables 1 and 2; ESIMS (+) m/z 545 [M + Na]+; HRESIMS (+) m/z 545.1789 [M + Na]+ (calcd for C29H30NaO9, 545.1782). (P,7R,8S,8′S)-Longipedlignan B (2): yellow, amorphous powder; [α]20 D −15 (c 0.1, MeOH); UV (MeOH) λmax (log ε) 210 (2.22), 222 (2.23), 259 (1.54), 283 (1.23) nm; ECD (c 0.1, MeOH) [θ] (nm) −21 × 103 (245), +6.7 × 103 (210); IR (KBr) νmax 3405, 2920, 2851, 1717, 1612, 1583, 1508, 1465, 1428, 1372, 1282, 1252, 1139, 1109, 1069, 1025, 977, 936, 830, 726 cm−1; 1H and 13C NMR data, see Tables 1 and 2; ESIMS (+) m/z 545 [M + Na]+; HRESIMS (+) m/z 545.1781 [M + Na]+ (calcd for C29H30NaO9, 545.1782). (P,7R,8S,8′R)-Longipedlignan C (3): yellow, amorphous powder; [α]20 D −40 (c 0.5, MeOH); UV (MeOH) λmax (log ε) 217 (2.23), 259 (2.33), 279 (1.86) nm; ECD (c 0.5, MeOH) [θ] (nm) −0.4 × 103 (297), −3.7 × 103 (254), +3.3 × 103 (203); IR (KBr) νmax 3486, 2935, 1709, 1616, 1507, 1462, 1427, 1371, 1341, 1248, 1202, 1172, 1139, 1106, 1067, 977, 936, 829, 770 cm−1; 1H and 13C NMR data, see Tables 1 and 2; ESIMS (+) m/z 571 [M + Na]+; HRESIMS (+) m/z 571.1951 [M + Na]+ (calcd for C31H32NaO9, 571.1939). (P,7R,8S,8′S)-Longipedlignan D (4): yellow, amorphous powder; [α]20 D −67 (c 0.5, MeOH); UV (MeOH) λmax (log ε) 217 (4.75), 259 (4.36), 279 (4.37) nm; ECD (c 0.1, MeOH) [θ] (nm) −0.5 × 103 (295), −2.6 × 103 (254), +1.8 × 103 (209); IR (KBr) νmax 3398, 2934, 2849, 1715, 1593, 1510, 1463, 1430, 1372, 1343, 1238, 1201, 1164, 1139, 1106, 1007, 982, 939, 832, 771 cm−1; 1H and 13C NMR data, see Tables 1 and 2; ESIMS (+) m/z 571 [M + Na]+; HRESIMS (+) m/z 571.1938 [M + Na]+ (calcd for C31H32NaO9, 571.1939). (P,7R,8S,8′R)-Longipedlignan E (5): yellow, amorphous powder; [α]20 D +16 (c 0.1, MeOH); UV (MeOH) λmax (log ε) 202 (1.01), 220 (1.05) nm; ECD (c 0.2, MeOH) [θ] (nm) −2.1 × 103(250), +4.2 × 103 (212); IR (KBr) νmax 3558, 3375, 2975, 2937, 1722, 1613, 1585, 1510, 1468, 1429, 1373, 1286, 1231, 1138, 1118, 1072, 1049, 1008, 975, 933, 829 cm−1; 1H and 13C NMR data, see Tables 1 and 2; ESIMS (+) m/z 507 [M + Na]+; HRESIMS (+) m/z 523.1942 [M + Na]+ (calcd for C27H32NaO9, 523.1939). (7R,8S,2′S,8′R)-Longipedlignan F (6): yellow, amorphous powder; [α]20 D +46 (c 0.1, MeOH); UV (MeOH) λmax (log ε) 202 (4.36), 221(4.64), 325(3.43) nm; ECD (c 0.1, MeOH) [θ] (nm), +0.8 × 103 (372). −1.7 × 103 (319), −1.7 × 103 (220); IR (KBr) νmax 3500, 2981, 2940, 1722, 1650, 1576, 1503, 1486, 1451, 1394, 1261, 1210, 1128, 1107, 1068, 1026, 960, 718 cm−1; 1H and 13C NMR data, see Tables 3 and 4; ESIMS (+) m/z 543 [M + Na]+; HRESIMS (+) m/z 521.1806 [M + H]+ (calcd for C29H28NaO9, 521.1806). (7R,8S,2′S,8′S)-Longipedlignan G (7): yellow, amorphous powder; [α]20 D −16 (c 0.1, MeOH); UV (MeOH) λmax (log ε) 220 (1.69), 320 (0.95); ECD (c 0.5, MeOH) [θ] (nm), +0.5 × 103 (373), −1.2 × 103

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.jnatprod.7b00934. 1 H NMR, 13C NMR, HMBC, HSQC, 1H−1H COSY, ROESY, ECD, and HRESIMS data for compounds 1− 10, computational details, and the detailed experimental conditions for the hepatoprotective assay (PDF)

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AUTHOR INFORMATION

Corresponding Authors

*Tel/Fax: +1-843-876-2316. E-mail: [email protected]. *Tel/Fax: +86-10-63164628. E-mail: [email protected]. ORCID

Peicheng Zhang: 0000-0002-0739-0019 Daneel Ferreira: 0000-0002-9375-7920 Robert J. Doerksen: 0000-0002-3789-1842 Shuai Li: 0000-0001-5721-8658 Author Contributions ∥

J. Liu and P. Pandey contributed equally to this work.

Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This study was supported financially by the CAMS Initiative for Innovative Medicine (CAMS-I2M-1-010). P.P. and R.J.D. were supported in part by grant P20GM104932 from the U.S. K



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National Institutes of Health (NIH) and National Science Foundation (NSF) Major Research Infrastructure grant 1338056; their research was conducted in part at the Mississippi Center for Supercomputing Research and in part in a facility constructed with support from research facilities improvement program C06RR14503 from the NIH.

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